Earth Planets Space, Vol. 56 (No. 3), pp. 353-357, 2004LETTER
University of Tokyo, Japan
(Received December 8, 2003; Revised February 25, 2004; Accepted February 25, 2004)
The 2003 Tokachi-oki earthquake of M 8.0, which occurred off the south-east coast of Hokkaido, Japan, was preceded by noticeable deep seismicity in the subducting slab, including a deep-focus earthquake of M 7.1. The 1952 Tokachi-oki earthquake of M 8.2 and the 1993 Kushiro-oki earthquake of M 7.5, which occurred also off the south-east of Hokkaido along the Kurile Trench, were also preceded by high deep seismicity including large deep-focus earthquakes of the M 7 or over (focal depths 200-600 km). The relations between the deep seismicity and the 'shallow' large earthquake (focal depth 100 km) in these three cases are quite similar. This result strongly supports the author's view (Mogi, 1973, 1988) that large 'shallow' earthquakes along the subduction zones are sometimes preceded by the occurrence of large deep-focus earthquakes.
Key words: Tokachi-oki, Kushiro-oki, deep-focus foreshocks, downward migration, subducting slab, trigger.